Reach down and scoop up some soil. Cupped in your hands may be 5000 different kinds of creatures—and as many individual cells as there are humans on the globe. That random handful might hold microscopic fungi, decomposing plant matter, a whisker-size nematode munching on the fungi, and a predatory, pinhead-size mite about to pounce on the nematode. One bacterium may fend off another with a potent antibiotic. It’s a whole world of often overlooked biodiversity.
Today, on the eve of World Soil Day, the Food and Agriculture Organization of the United Nations has released its first ever global assessment of the biodiversity in this underground world. Some 300 experts have pooled their knowledge and data to describe the diversity of these organisms, the roles they play in both natural and agricultural environments, and the threats they face.
“The organisms below ground are arguably just as important, if not more important, than what’s above ground,” says Noah Fierer, a soil ecologist at the University of Colorado, Boulder, who did not contribute to the report. The report details how they boost crop growth and purify soil and water. Together with plant root systems, these organisms store more carbon, potentially for longer, than the aboveground parts of trees do. “Depending on how we handle soil, it could become a help or a burden to face the crisis of biodiversity or climate change,” says Francisco Pugnaire, a soil and plant ecologist at the Spanish National Research Council’s Experimental Station of Arid Zones.
Yet with each pass of the bulldozer or tractor, each forest fire, each oil spill, even the constant traffic of hikers along a popular trail, more and more soil organisms are being killed off. By compiling research on these subterranean ecosystems and how they affect visible ones, the report’s authors hope to convince scientists, policymakers, and the general public to take steps to slow this loss.
“You just can’t have a Mars-like soil and expect to maintain the food supply and forests,” warns Diana Wall, an ecologist at Colorado State University who contributed to the report. Current conservation efforts are not helping much, she adds. For example, soil biodiversity hot spots aren’t necessarily in the same place as the biodiversity hot spots that conservationists focus on. “We are managing [conservation] by what we see above ground, which doesn’t necessarily match what’s below ground.” In contrast, Fierer says, “If you preserve the soil, you will likely preserve the whole ecosystem.”
Soil is a mix of organic material, minerals, gases, and other components that provide the substrate for plants to grow. About 40% of all animals find food, shelter, or refuge in soil during part of their life cycle.
Scientists have mostly focused on the largest and smallest soil creatures. For centuries, natural historians have observed ants, termites, and even earthworms and moles that chew, wiggle, and dig their way among soil’s particles, some feasting on decaying leaves and other debris and some feasting on each other. Those ecosystem engineers aerate the soil and create underground passageways that make soil more amenable for other life. And over the past several decades, microbiologists sequencing soil DNA have discovered an astonishing diversity of bacteria and fungi, which process that litter into organic material.
But in between the scales of macroscopic animals and microbes lie thousands of long-overlooked tiny creatures—the micro- and meso-fauna. Microscopic protists, nematodes, and tardigrades inhabit the watery films surrounding soil particles. Slightly larger animals up to 2 millimeters in size, such as mites, springtails, and insect larvae, live in the airy pores between those particles, helping make soil one of the most biologically diverse habitats on Earth. “How little we know [about these creatures] is a bit overwhelming,” Fierer says.
This diversity creates a rich, complex ecosystem that boosts crop growth, breaks down pollutants, and can serve as a nearly inexhaustible sink for carbon. Some soil organisms promote plant diversity and many have yielded important compounds, from antibiotics to natural pesticides. “Without soil organisms and the activities they carry out, it would be impossible for other organisms to survive,” says Stephen Wood, a soil ecologist at the Nature Conservancy.
Hidden below ground, these ecosystems seemed immune to aboveground disturbance, Wood says. “For a long time, soil scientists thought that soil microorganisms were so well spread around the world that land management would not harm them,” he explains. “We now know that soil microorganisms can be very specific to very specific habitats and species,” habitats that are rapidly disappearing as farms and cities expand.
The report lists a dozen human activities taking a major toll on soil organisms. They include deforestation, intense agriculture, acidification due to pollutants, salinization from improper irrigation, soil compaction, surface sealing, fire, and erosion. “If you pave over a site, you are sealing off an entire belowground ecosystem,” Fierer says. “And that’s happening all over the globe.”
A few governments and companies are making some progress. Several states are considering legislation that would help protect soils from destructive human activities. In China, the Agricultural Green Development program works to conserve soil by avoiding tilling and by interweaving different crops to preserve biodiversity. However, “Most organizations want to protect soil biodiversity as a means to an end—[to] the benefit of people and/or nature,” Wood points out.
Some researchers hope the report will encourage protecting soil organisms for their own sake. “Soil biodiversity is huge, and we must not destroy it without knowing what potential there is for improving sustainability,” says Mary Scholes, a biogeochemist at the University of the Witwatersrand.
Fierer thinks the new assessment will also awaken a sense of wonder. “My hope is people will look at this document and say ‘Huh, I never thought about [soil organisms] and all the things they do for me.’”